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Tilt problem?

Canon EOS 6D Flattener Acquisition Deep Sky Tilt
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l.roulin avatar
l.roulin avatar
Hi everyone,

Ever since I started astrophotography – not too long ago –, I’ve been dealing with bad star shapes on a large portion of my subs. It is not to be unexpected, as I’m using a full-frame camera with an optical train not meant to accomodate it – modded Canon 6D, TS Optics SD Apo 72 f/6 and TSFlat72. I’m suspecting a tilt problem, because the stars are being stretched vertically on the left of the frame, and horizontally on the right (see first two images, open in a new tab to zoom in). I did apply some electrician tape between the T-ring and the camera body, one one side, as it wasn’t a snug fit, and it seemed to have somewhat mitigated the problem, but it remains there.



After that, it occurred to me that I didn’t use the flattener’s rotator lately. So I went back to look at subs I took this winter, where I did move the rotator to frame IC 434 (next image). And there it was, I noticed that the elongated stars rotated slightly as well. As the lenses of the flattener move along the rotator, I guess it is possible that at least part of the problem resides in the objective lenses. Do you have any idea of what might cause what I’m seeing, and some tests I can run next time the clouds decide to clear?

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Frank Szabo avatar
Frank Szabo avatar
TS Optics SD Apo 72 f/6 --

I just googled this scope and this has to be same scope as the Astrotech 72 EDII, looks identical, beside the paint job. 
https://www.astronomics.com/astro-tech-at72edii-refractor-ota-fpl-53-f-6.html

I have a similar tilt and star shape issues going on with my AT72 EDII,  I can almost guarantee you that it's your scope.  I can't tell if both the reducer and the objective lens, but I have tried other flatteners with the telescope and none could flatten the image, beside its own and I have already sent mine back for a replacement, only to see the same issue again. 

Ever since noticed the issues, I keep telling people to avoid this telescope, it's just junk.  Basically, I just avoid any telescope now that is indicated as 70mm or 72mm, because I'm afraid to buy a lemon. 

They must have a serious miscalculation going on with that scope, because even the cheaper AT60  (60mm F6 doublet)  is a better telescope. 
I have no bias against any brands or telescope types, but truth to be told, even my Skywatcher Evoguide 50DX produces better images than my AT72.  And that's $200 cheaper. 
Look at my thread here, I am utterly disappointed with this telescope which isn't even cheap to begin with: 
https://www.astrobin.com/forum/c/astrophotography/deep-sky/maybe-im-asking-too-much-from-this-telescope-at72edii/

This is my "best" image to the date with the AT72, the lower left corner the stars are elongated, while on the mid right side the stars are circling and oval shaped.  What's worse is, this is a tiny sensor of the 3008x3008 sensor of the ASI533  I can't and I don't even want to see what would an APS-C size image look like. 
This is not a backspacing issue, I tried 3-4 different settings with the backfocus and this is the best I can make it to be. The stars are also bloating too much.

Frank Szabo avatar
Frank Szabo avatar
If you don't mind, I downloaded your image and ran the ASTAP inspection on the image. You got a severe tilt. 

Look: 
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Frank Szabo avatar
Frank Szabo avatar
In typical circumstances the HFD value map should show something like the center part being a bit darker, but the less greyshade variation the better. Yours is tilted sideways so the HFD values decrease to the left (darker) and high on the right (brighter).   I don't know any way to fix a tilt on a telescope itself, most people end up fiddling with their camera, buying a tilt corrector, even though the tilt may caused by the objective lens and/or the flattener or other components in the imaging train. 

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l.roulin avatar
l.roulin avatar
Hi Frank,

This model is indeed, as far as I know, the same as the AT72EDII.

Having had a look at your image, it seems you have additional issues, maybe pinched optics, as someone pointed out in your thread. I see you did take measures to alleviate it, but to no effect. How does a star look at high power, in focus?

What we have in common is the star bloat, which I fully expected, knowing it is a short doublet. I bought it initially for visual only, for which it performed decently. Too much spherical aberration for my taste, but again, it is a short doublet. I accept this shortcoming, as the scope is what I had laying around when I started imaging.

Now regarding what I suspected - the tilt: this is what Siril's analyzer shows me indeed. What I don't know is the source of it. Probably between the objective cell and the flattener, as I think it slightly rotates when moving the flattener's rotator. If only I had some clear sky to test a more pronounced rotation… Then I guess the only workaround would be to use a tilt plate.
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